1. Field of the Invention
The present invention relates to an electronic system having a resistance circuit in which a plurality of resistors are serially connected with each other.
2. Description of Related Art
As an electronic system capable of suppressing the increase in temperatures of electronic parts, an electronic circuit device has been disclosed in Published Japanese Patent First Publication No. 2010-73943. This device is provided with an electronic circuit chip, a base substrate and a thermal conductivity member. The chip is disposed on the substrate. The conductivity member is disposed between the chip and the substrate while being in contact with the chip and the substrate. Therefore, the conductivity member reduces thermal resistance in the route from the chip to the substrate. Accordingly, heat generated in the chip can be efficiently radiated to the outside, and increase in the temperature of the chip can be suppressed.
Hybrid vehicles have a motor control system in which a high voltage of a direct current source insulated from the vehicle body is changed to an alternating current voltage to apply this alternating current voltage to a vehicle driving motor. The control system is provided with a smoothing capacitor to smooth the high direct current voltage. When the operation of the control system is started, electric charge supplied from the source is accumulated in the capacitor so as to smooth the high direct current voltage. Thereafter, when the operation of the control system is ended, the source is disconnected from the capacitor, and the charge accumulated in the capacitor is maintained. Therefore, there is a probability that a person coming into contact with the capacitor receives an electric shock. To avoid this problem, under control of a microcomputer of the system, reactive power is supplied to the motor so as to generate no torque in the motor. Therefore, the charge accumulated in the capacitor is discharged. Further, in preparation for a failure occurring in the microcomputer, the control system is provided with a resistance circuit, and charge of the capacitor is discharged through the resistance circuit.
More specifically, the resistance circuit is disposed in parallel to the capacitor so as to always discharge charge of the capacitor at a low rate. To install the resistance circuit on a base substrate, a plurality of resistors having the same resistance values are aligned in a line on the substrate and are serially connected with one another as the resistance circuit. When the discharging current flows through the resistance circuit, each resistor generates heat. Therefore, temperature of each resistor is inevitably increased. Because heat radiating conditions of each resistor depend on the position of the resistor on the substrate and/or the relative position of the resistor to the resistance circuit, temperature of the resistor located at a low heat radiating position is considerably increased.
To suppress the increase in temperatures of the resistors, the structure of the electronic circuit device is applied for the control system. More specifically, a thermal conductivity member is disposed between the resistance circuit and the substrate while being directly in contact with bodies of the serially-connected resistors and the substrate. To heighten the thermal conductivity of the member, a metallic material such as copper is generally used. However, because bodies of the serially-connected resistors are in contact with the same metallic member, the insulation of each resistor from the other resistors cannot be obtained. That is, the motor control system cannot secure the distance for insulation among the resistors of the resistance circuit receiving high voltage.